1. Field of the Invention
The present invention relates to a small solid-state laser device capable of producing a stable short wavelength output, or more in particular to an optical information processing system such as an optical disk apparatus or laser beam printer utilizing the solid-state laser device as a light source.
2. Description of the Prior Art
A study is under way to shorten the wavelength of a light source of an optical information processing system for achieving high-density recording on a recording medium. When a solid-state laser using a solid material for a laser crystal is compared with a semiconductor laser in regard to the reflection noise, for example, the output of the solid-state laser device is more stable than that of the semiconductor laser in view of the facts that (1) the semiconductor laser has a lower reflection factor of a resonator mirror and is more liable to be coupled with an external optical system, that (2) the resonator of the semiconductor laser is generally shorter in length than that of the solid-state laser, and therefore the oscillation mode is more likely to be single, thereby leading to a higher coherence, and that (3) the laser gain distribution and refractive index of the semiconductor laser are subject to greater fluctuations due to the temperature and other conditions, with the oscillation wavelength being unstable.
JP-A-3-49278 (U.S. Pat. No. 4,933,947) discloses system comprising a resonator of a solid-state laser for irradiating pumping power on a solid laser crystal and a nonlinear optical crystal arranged in the resonator for generating second and higher harmonic waves of the solid laser crystal. As a laser crystal used for such a solid-state laser device, Nd:YVO.sub.4 (neodymium yttrium vanadate) in place of Nd:YAG (neodymium YAG) has recently been closely watched. This crystal is described in IEEE Journal of Quantum Electronics, Vol. 26, No. 9 pp. 1451-1459 (Sep. 1990). Nd:YVO.sub.4, as compared with Nd:YAG, has the feature that the threshold value of laser oscillation is low and the absorption length short, and therefore is liable to oscillate in a single mode. As a result, on the assumption that the laser beam generated from Nd:YVO.sub.4 as a solid laser crystal is used as a fundamental wave with a second high harmonic or a sum frequency generated through a nonlinear optical crystal such as KTP (KTiOPO.sub.4) (potassium titanate phosphate) placed in the resonator or KN (KNbO.sub.3) (potassium niobate), then a wavelength conversion efficiency higher than with a solid-state laser using only a laser crystal material (such as Nd:YAG) is obtained.